Electric resistance element



Patented Aug. 21, 1923.

FRANK L. DRIVER, JR., 0F NEWARK. NEW JERSEY, A'SSIGNOR T0 DRIVER-HARRISCOMPANY, OF HARRISON, NEW JERSEY, A CORPORATION OF NEW JERSEY.

ELECTRIC RESISTANCE ELEMENT.

No Drawing.

To all wlzom it may concern:

Be it known that I, FRANK L. DRIVER, J r., a citizen of the UnitedStates, residing at Newark, county of Essex, State of New Jer sey, haveinvented a certain new and useful Improvement in Electric ResistanceElements, of which the following is a full. clear, and exactdescription.

My invention relates to electric resistance elements and has for itsobject to produce an electric resistance element formed of wire whichwill have, when in use, a longer life than those now available. Itfurther has for its object to produce a' drawn electric resistanceelement in which the formation of hot spots when in use is largelyeliminated. It further has for its object to produce a drawn electricresistance element in which the coefficient of expansion of the surfacewhen oxidized shall be substantially the same as the coefficient ofexpansion of the body of the element.

In the manufacture of wire for electric resistance elements .forelectrical heating and cookin devices there are two main matters to beconsidered. The first is the wire must have a high specific electricalresistance and the second is that the wire must' have a long life andhigh resistance to oxidation at the operating temperature. There havebeen on the market for some years several nickel-chromium-iron andnickelchromium resistance wires which combine comparatively highspecific electrical resistance with comparatively long life under hightemperatures. These alloys have 'ven satisfaction to a limited extent,being t e best known alloys for this purpose, but the electrical heatingandcooking device manufacturers are demandin a wire for electricalresistances which wi I operate under more severe conditions than anywire heretofore attainable and at the same time continue to functionproperly over a greater number of operating hours. A great deal ofeffort has been put forth by me with the idea of finding some means ofeither increasing the life of the present alloys when used for thispurpose or of discovering new alloys to take the places of the oldalloys. This effort has been extended along the lines of research workwhich involved the alloying of various new metals with thenickelchromium and nickel-chromium-iron alloys already used. In thiswork I have discov- Application filed December 6, 1921. Serial No.520,248.

end that the metal silicon has apparently been greatly under-rated as ameans fOIlIH- proving the nickel-chromium and nickelchromium-iron alloysfor resistance wire. I have discovered that by the addition of siliconto such alloys their resistance to oxidation can be increased, that theformation of hot spots can be greatly reduced, if not eliminated, andthat the coefficients of expansion of any oxidized surface formedthereon and the mass of the wire can be made substantially equal. Allthese features tend to prolong the life of the resistance element andresult in a very superior article.

I have further discovered that it is commercially possible by the usualmethods to manufacture drawn resistance elements of these alloys whensilicon has been added thereto; that is, that the hammering of thelngot, the hot rolling of the rod, and the cold drawing of the wire iscommercially possible.

I have found that very sinall amounts of silicon materially increase thespecific electrical resistance of the nickel-chromium andnickel-chromium-iron alloys, and also ma terially increase their lifeunder operating conditions. I have found that the most beneficialresults are to be obtained when the silicon is from about .5 to 1.5 percent. by weight in the alloy, although I have found that even smalleramounts of silicon increase the life of the wire. When a larger amountof silicon is used there is no apparent increase in resistance tooxidation. The difficulty of mechanically working the alloy and drawinginto wire is, however, greatly increased, although it is possible todraw such wire when the amount of silicon is approximately 10 per cent.

One of the great advantages accruing from the use of silicon inaccordance with my invention is that the wire when drawn and tested on alife testing machine at temperature ranging about 1000 C. shows an ithas a coefficient of expansion materially different from the coefficientof expansion of the mass is dislodged during the cooling. The additionof the element silicon in accordance with my invention as one of themain constituents of the alloy has the effect of bringing the twocoeflicients of expansion into substantial equilibrium so that the oxideadheres very closely under all working conditions of temperature andrenders the wire substantially immune from further oxidation anddeterioration.

Among the wires containing nickel, chro mium and iron which I havetested were those containing the following percentages of silicon:

These wires tested better than wires containing similar proportions ofnickel, chromium and iron but no silicon in the ratio of two, three andsix, showing that the increase in life was substantially in directproportion to their silicon content.

By the addition of the element silicon the yield point and ultimatetensile strength of the wire is greatly increased, due to the fact thata denser and more homogenous metal results. This increased density andhomogeneity results from the fact that any carbide in the mass duringthe solidifying of the molten metal into an ingot is held in moltencondition until the alloy has substantially reached the solidifying orfreezing stage, this being due to the fact that the freezing point ofthe alloy is lowered by the addition of the silicon. This, together withthe formation of nickel silicides results in a more even distribution ofany carbide formed and thus prevents segregation of such carbide. In theabsence of means for preventing such segregation any carbide presentsolidifies or freezes out first during the cooling of the alloy, takingthe form of long needle-shaped crystals segregated in groups located atintervals throughout the alloy. The presence of such crystals when theyexist is much in evidence, since the carbides being harder than thesolid solution of nickel-chromiuni or nickel-chron'iium iron in whichthey are found. stand out in relief in any polished section of thealloy. This relief may be further accentuated by etching the polishedsurface with any proper etching reagent. These crystals being of lowerconductivity increase the resistivity of the wire at the point at whichsegregations occur. In consequence these points become more highlyheated when a. current is passed through the wire, producing the hotspots which have been previously referred to.

As is well understood, etficient resistance wires containing nickel andchrominum sometimes contain substantial amounts of iron and sometimesare free from iron. The iron when present is of advantage in reducingthe cost of commercial manufacture. So far as the ultimate product isconcerned, it is preferably omitted.

In both of these alloys when commercially manufactured some carbon isnecessarily present. Where iron is not present carbon 'exists in theform of chrominum carbide, probably Cr C When iron is also present thecarbon exists in the form of a complex iron-chromium carbide, probablyCr,C Fe C. These carbides are soluble only to a limited extent in thesolid state and it is of great importance to have the portions which arenot in solution equally distributed throughout the mass, so as to makethe mass as dense and homogeneous as possible and this result Iaccomplish by my addition of silicon.

The silicon forms with the nickel, silicides of nickel which remain insolid so1ution in the metal and there is no evidence from microscopicinvestigation that these silicides separate at all from the main body ofthe alloy on cooling. Their presence, however, appears to bring about abetter distribution of the insoluble carbides of chrominum or iron andchromium, which may be present, resulting in the absence of hot spotsduring the operation and the material prolongation of the life of theresistance element.

In order to embody my invention in connection with acommercial alloycomposed of approximately 90 parts nickel and 10 parts chromium andcarbon varying from .5 to 2 per cent, I preferably add to the mixturebetween 1 and 2 per cent of silicon by weight, although this may bevaried from .5 to 10 per cent. hen a portion of the nickel is replacedby iron such for instance as in an alloy containing nickel 60 per cent,iron 26 per cent, chromium 12 per cent, in which, in commercialmanufacture, there would be carbon from .15 to 2 per cent, I makeadditions of silicon, namely, preferably between 1 and 2 per cent byweight, although the percentage may vary from .5 to 10 per cent,resulting in each case in an alloy which is dense and homogeneous, thecarbides not being segregated but being dis tributed throughout the massand the coefficient of expansion of any surface oxide which may beformed being substantially the same as the coefficient of expansion ofthe body of the mass.

As will be evident to those skilled in the art, my invention permits ofvarious modifications Without departing from the spirit thereof or thescope of the appended claims.

What I claim is:

1. A drawn metallic resistance element for high temperature heatingcomposed of a carbide containing alloy comprising nickel and chromium,the carbon content being at least two tenths per cent (0.20%) andproducing free carbide, and a carbide distributing constituent, all freecarbide being distributed with substantial uniformity throughout themass.

2. A drawn metallic. resistance element composed of a carbide containingalloy comprising nickel, chromium and silicon, the carbon content beingat least 0.20 per cent, the silicon being sufficient to causesubstantially all of any carbide in the alloy to be distributed withsubstantial uniformity throughout the mass.

3. A drawn metallic resistance element composed of a carbide containingalloy comprising nickel, chromium and silicon the carbon content beingat least 0.20 per cent, the silicon being between .5' per cent and 10per cent by Weight of the mass.

4. A drawn metallic resistance element composed of a carbide containingalloy comprising nickel, chromium and SlllCOIl the carbon content beingat least 0. 20 per cent, the silicon suflicient to cause the coeflicientof the body of the element tobe substantially equal to the coeflicientof expansion of the surface when oxidized.

5. A drawn metallic. resistance element composed of a carbide containingalloy comprising nickel, chromium and silicon, the carbon content beingat least 0.20 per cent, the silicon being between 1 per centand 2 percent by wei ht of the mass.

F ANK L. DRIVER, JR.

